1. Field of the Invention
This invention relates to coating compositions for glass fibres, more particularly glass fibres which are intended for use as reinforcement in cement products. In the alkaline environment of a normal Portland cement, which is mainly due to the presence of lime (calcium hydroxide), fibres of generally available glass compositions, such as that widely known as E-glass, are rapidly attacked and weakened so that the additional strength imparted to the cement by the glass fibres is rapidly lost.
2. Description of the Prior Art
Various alkali-resistant glass compositions have been devised which retain their strength better in cement.
Thus British Patent Specification No. 1,200,732 (National Research Development Corporation) describes and claims a composite fibre/cementitious product comprising fibrous reinforcing material distributed throughout a cement matrix, in which the reinforcing material is primarily a glass having per se a degree of alkali resistance such that when tested in the form of an abraded fibre of length 2 1/2 inches and diameter of from 0.4 to 1.0 .times. 10.sup.-3 inches said fibre has a tensile strength of at least 100,000 p.s.i. after treatment with saturated aqueous Ca(OH).sub.2 solution at 100.degree. C for 4 hours followed by successive washings at ambient temperature with water, then with aqueous hydrochloric acid (1%) for 1 minute, water, acetone, followed by drying, said fibre experiencing not more than 10% reduction in diameter during said test.
British Patent Specification No. 1,243,972 (N.R.D.C.) discloses and claims such composite fibre/cementitious products in which the glass contains at least 65% SiO.sub.2 and at least 10% ZrO.sub.2 by weight. Patent Specification No. 1,243,973 (N.R.D.C.) discloses and claims alkali-resistant glass fibres derived from a glass containing, in weight percentages, 65 - 80% SiO.sub.2, 10 - 20% ZrO.sub.2 and 10 - 20% of a network modifier which is an alkali metal oxide, an alkaline earth metal oxide or zinc oxide, said glass being one which has a tensile strength as set out above.
Further ranges of glass compositions for forming alkali-resistant glass fibres are disclosed and claimed in our British Patent Specification Nos. 1,290,528 and 1,389,019. No. 1,290,528 claims glass compositions for forming glass fibres which are to be incorporated as reinforcement in cementitious products, comprising, in molecular weight percentages:-
______________________________________ SiO.sub.2 62% to 75% ZrO.sub.2 7% to 11% R.sub.2 O 13% to 23% R'O 1% to 10% Al.sub.2 O.sub.3 0% to 4% B.sub.2 O.sub.3 0% to 6% Fe.sub.2 O.sub.3 0% to 5% CaF.sub.2 0% to 2% TiO.sub.2 0% to 4% ______________________________________
wherein R.sub.2 O represents Na.sub.2 O, up to 2 mol.% of which may be replaced by Li.sub.2 O, and R'O is an oxide selected from the group consisting of the alkaline earth metal oxides, zinc oxide (ZnO) and manganous oxide (MnO), the balance if any consisting of other compatible constituents.
Pat. No. 1,389,019 claims glass compositions for use in the production of alkali-resistant continuously-drawn glass fibres, comprising in molar percentages on the oxide basis:-
______________________________________ SiO.sub.2 67 to 82 ZrO.sub.2 7 to 10 R.sub.2 O 9 to 22.5 F.sub.2 3 to 9 Al.sub.2 O.sub.3 0 to 5 (computed as AlO.sub.1.5) ______________________________________
where R = Na, up to 5 mol.% of which may be replaced by Li or K, and the fluorine is included in substitution for oxygen in one or more of the oxides, the maximum value of the molar percentage represented by SiO.sub.2 + ZrO.sub.2 + AlO.sub.1.5 being on a sliding scale dependent on the content of ZrO.sub.2, ranging, when F.sub.2 = 9 mol.%, from 89 mol.% when the ZrO.sub.2 content is 7 mol.%, to 88 mol.% when the ZrO.sub.2 content is 8.5 mol.% down to 87 mol.% when the ZrO.sub.2 content is 10 mol.%, the said maximum value being reduced by a further 5 mol.% over the whole scale when F.sub.2 = 3 mol.%.
South African Patent Specification No. 73/4476 (Owens-Corning Fiberglas Corporation) describes another range of alkali-resistant glasses, and glass fibres made from them, having compositions within the following range:
______________________________________ Weight Percent Mole Percent ______________________________________ SiO.sub.2 60 to 62 65 to 67 CaO 4 to 6 4.5 to 6.5 Na.sub.2 O 14 to 15 14.5 to 16 K.sub.2 O 2 to 3 1 to 2.5 ZrO.sub.2 10 to 11 5 to 6 TiO.sub.2 5.5 to 8 4.5 to 6.5 ______________________________________
Although alkali-resistant glass fibres as described in the above Patent Specifications retain their strength in cement much better than fibres of conventional glasses, such as E-glass, there is nevertheless, a gradual deterioration over long periods.
When producing continuous glass fibres for any purpose, it is normal practice to coat the individual continuously drawn glass fibres immediately after drawing, with a size composition which provides a mechanical protection and a lubricant for the fibres to minimise breakage and abrasion during subsequent handling, such as the bringing together of numerous individual fibres to form a strand and the winding of the strand on a spool or drum. The size compositions previously used on glass fibres to be included in a cementitious matrix do not have any material effect on the long term resistance of the glass to attack by the alkalis in cement.
Protective coating compositions have also been applied to glass fibres at various stages in their production and handling, and it has, for example, been proposed to use a furane resin in such a coating for increasing the alkali resistance of the glass fibre material to render it suitable for use in reinforcing concrete.